Since the lithium-ion battery has been commercialized, due to advantages, such as a high energy density, a high operating voltage, none memory effect and a long cycle life and the like, the lithium-ion battery is widely used as a power supply for various mobile devices. With large scale applications of the lithium-ion battery, the safety problem of the lithium-ion battery is increasing significantly.
The lithium-ion battery mainly comprises a positive electrode plate, a negative electrode plate, a separator and an electrolyte. The separator is provided between the positive electrode plate and the negative electrode plate, and mainly functions to: physically isolate the positive electrode plate and the negative electrode plate of the lithium-ion battery so as to prevent an interior short circuit between the positive electrode plate and the negative electrode plate, ensure the lithium ions to pass the electrolyte and move back and forth uniformly and freely between the positive electrode plate and the negative electrode plate, and cut off the channel of the lithium ions to prevent the further thermal runaway due to micropore self-shutdown ability of the separator at a too high temperature.
Three-layered polypropylene/polyethylene/polypropylene composite separator can more effectively provide a thermal shutdown compared with a single-layer polyethylene separator or a single-layer polypropylene separator. Herein, the polypropylene layer functions as a support framework, when the interior temperature of the lithium-ion battery rises close to the melting point of PE, the micropores in the polyethylene layer will be shut down, thereby cutting off the channel of the lithium ions. A temperature range of the thermal shutdown is substantially between the melting point of the polyethylene layer and the melting point of the polypropylene layer, and is about 140° C.˜160° C. However, many safety tests have shown that: a more serious safety hazard will occur when the temperature of the lithium-ion battery rises to be equal to or more than 100° C., and the temperature of the shutdown of the polyethylene layer is high as 130° C., when the temperature of the lithium-ion battery reaches 130° C., the shutdown of the polyethylene layer cannot effectively function as a safety protection. And the safety performance of the single-layer polyethylene separator and the single-layer polypropylene separator are worse.
In practice when the lithium-ion battery is abused (such as overcharge, thermal shock and puncture and the like), the temperature of the lithium-ion battery generally will rise to be equal to or more than 90° C., once the interior temperature of the lithium-ion battery is more than 90° C., the conventional polyethylene separator, polypropylene separator will have a more serious thermal shrinkage, a short circuit would be established between the positive electrode plate and the negative electrode plate and more heat would be generated, the lithium-ion battery would be easily fired or even exploded. Regarding this situation, a person skilled in the art generally will coat a layer containing ceramics onto the separator so as to reduce the thermal shrinkage and prevent the short circuit between the positive electrode plate and the negative electrode plate from being established, which however does not work on the abuses which more easily happen (such as overcharge and the like), this is because the positive electrode plate and the negative electrode plate of the lithium-ion battery will generate large amount of heat under the situation of overcharge, even if the ceramics separator does not shrink, the overcharge still continues, and the capacity continues to increase, the voltage accordingly rises, finally the heat generated from the reactions between the electrode plates and the electrolyte becomes too large, the lithium-ion battery will be fired or even exploded. In order to ensure the safety under the situation of the overcharge, the channel of the lithium ions must be cut off when the lithium-ion battery is overcharged, so that the overcharge would be stopped, and in turn the safety of the lithium-ion battery under the situation of overcharge would be ensured.